A type of transmission known as a "power shift" transmission is used in many on-road and off-road applications. In this type of transmission, a torque converter is usually disposed between the vehicle engine and the gear box. The gear box, which typically has automatic shift capability, differs from conventional automobile transmissions in that it uses multiple shafts that mount gear/clutch assemblies, as opposed to planetary units used in conventional automobile automatic transmissions. The gear clutches are selectively engaged to couple the gearbox input with the gearbox output in a ratio determined by the selected gears. This arrangement provides the multiple speed capability for the transmission.
This type of transmission typically includes a forward/reverse shaft which mounts forward and reverse clutch/gear assemblies. This type of transmission also includes at least one countershaft which mounts gears for providing other speeds and utilizes clutches for coupling the gears to the shaft. The forward and reverse clutches determine whether the transmission is in forward, reverse or in neutral (neither clutch engaged), whereas the selective application of the other clutches determine the "range" of the transmission. In transmissions that employ a torque converter, it is known to provide a "lock-up" clutch to mechanically lock the turbine to the pump impeller in order to eliminate slippage and, hence, energy loss when the vehicle is moving.
In conventional transmissions of this type, if four (4) forward speeds are desired, the gear box may be supplied with four pairs of meshing gears and four clutches for selectively engaging the appropriate gear pair. Accordingly, eight speed transmissions might require eight pairs of gears and eight clutches or two input clutches and four pairs of gears.
Transmissions of this type, depending on application, are provided in various configurations. In one configuration known as a "straight-through" configuration, the axis of the input shaft is coincident with an axis of the output shaft. In a "short drop" transmission, the axis of the output shaft is displaced vertically with respect to the axis of the input shaft. In a "long drop" transmission, the vertical distance between the output shaft and input shaft is even greater. In many, if not most instances, the construction and design of these three types of transmissions are substantially different even in situations where all three designs have substantially the same horse power rating. Each is individually designed to provide the necessary vertical distance between the input and the output. Manufacturers of these types of transmissions expend large amounts of dollars for transmission tooling unique to each type of transmission.